Lithium secondary batteries, which are one type of non-aqueous electrolyte batteries, have a high energy density, and thus are widely used as a power source for portable devices such as cell phones and notebook personal computers. With advances in the portable devices, the trend toward higher capacity lithium secondary batteries is further increasing, and therefore, ensuring safety is becoming an important issue.
Currently available lithium secondary batteries employ, as a separator interposed between a positive electrode and a negative electrode, for example, a polyolefin porous film having a thickness of approximately 15 to 30 μm. In order to ensure what is called a shutdown effect that causes a resin constituting a separator to melt so as to close the pores at a temperature equal to or less than a temperature at which thermal runaway occurs in the battery, and thereby increases the internal resistance of the battery so as to improve battery safety in the event of short-circuiting or the like, polyethylene having a low melting point is often used as a separator material.
As the separator, for example, a film that has been uniaxially or biaxially drawn so as to increase the porosity and improve the strength is used. Such a separator is supplied as a film that exists as a single film, and thus is required to have a certain strength in terms of workability. For this reason, the above-described drawing is performed to ensure a certain strength. However, because such a drawn film has an increased crystallinity, as well as an increased shutdown temperature close to the thermal runaway temperature of the battery, it cannot be said that the margin for ensuring battery safety is sufficient.
There is also another problem that distortion occurs in the film due to drawing, and if the film is exposed to a high temperature, the film shrinks due to residual stress. Because the shrinkage temperature is very close to the shutdown temperature, in the case of using the polyolefin porous film separator, it is necessary to prevent a temperature increase in the battery by reducing the current immediately when the battery temperature reaches the shutdown temperature in the event of overcharging or the like. If the pores do not close sufficiently, and the current cannot be decreased immediately, the battery temperature will quickly rise to the shrinkage temperature of the separator, causing a risk of ignition due to internal short-circuiting.
As techniques for preventing short-circuiting caused by such separator thermal shrinkage so as to enhance battery reliability, for example, Patent Documents 1 and 2 propose the use of a separator in which a heat resistant porous layer for increasing heat resistance is formed on the surface of a resin porous film containing a thermoplastic resin.
According to the techniques disclosed in Patent Documents 1 and 2, it is possible to provide a non-aqueous electrolyte battery having excellent safety and reliability in which thermal runaway does not easily occur even in the event of overheating.